Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crankshaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.
A free piston internal combustion engine likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines the combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed and an intermediate compression cylinder between the combustion cylinder and the hydraulic cylinder. The combustion cylinder has the largest inside diameter; the compression cylinder has an inside diameter which is smaller than the combustion cylinder; and the hydraulic cylinder has an inside diameter which is still yet smaller than the compression cylinder. A compression head which is attached to and carried by the plunger at a location between the piston head and plunger head has an outside diameter which is just slightly smaller than the inside diameter of the compression cylinder. A high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the compression cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the TDC position.
With a free piston engine as described above, a check valve interconnects a variable volume pressure chamber within the hydraulic cylinder with a high pressure hydraulic accumulator. As the piston passes the TDC position and begins toward the BDC position during a return stroke, the check valve is biased to an open position by the increasing pressure which is created within the pressure chamber of the hydraulic cylinder. The maximum pressure which can be created within the high pressure hydraulic accumulator is equal to the maximum pressure which is developed within the hydraulic cylinder. Since the check valve opens at or near the TDC position, the maximum pressure which is developed within the hydraulic cylinder corresponds to the pressure developed during a full stroke of the piston traveling from the TDC to is the BDC position.
Under certain operating conditions, it may be desirable to provide the free piston engine with a pressure output which is higher than normally attained. For example, certain operating conditions may require a high pressure but low flow supply of hydraulic fluid from the free piston engine. An example of such an operating condition would be when a front end payloader is digging into a mound of dirt and the hydrostatic drive within the payloader requires more pressure than is typically available from the free piston engine.
The present invention is directed to overcoming one or more of the problems as set forth above.